Process for case hardening metals



PROCESS FOR CASE HARDENING METALS William P. Roe, Westfieid, N. .L,assignor to National Lead Company, New York, N. Y., a corporation of NewJersey No Drawing. Application November 3, 1954 Serial No. 466,685

4 Claims. (Cl. 117-21) The present invention relates in general to amethod for treating metals and more especially to a superior method forcase hardening metals including metals of the fourth group of theperiodic table.

The process of forming a hard adherent metal coating on ferrous metalsby increasing the carbon content of the surface metal, and in some casesthe nitrogen content as well, and thereafter heat treating thecarburized surface has long been known in the art as case hardening. Bythis process a composite structureis obtained comprising a relativelysoft but strong and tough interior or core, and an extremely hardsurface coating. Commercial carburizing of ferrous metals is carried outby any one of several methods, including pack carburizing, gascarburizing, carbonitriding and liquid carburizing. Of these methods,that of pack carburizing is one of the oldest and most extensivelyemployed, the usual carburizing compound being a mixture of alkali ormetal carbonates, charcoal and a binder.

Among the more recent developments in the field of refractory metals hasbeen the commercial production of titanium andits alloys; This metalexhibits many properties which adapt it not only to specialty fields butto the more common construction uses. However, it has been observed thattitanium has a marked tendency to gall and seize under certainconditions of use, which characteristics seriously limit its commercialapplications; Moreover, while titanium metal has remarkably good wearand erosion resistance, it is advantageous to improve these propertiesas well as its resistance to certain types of corrosive attack at hightemperatures. 7

An object, therefore, of the present invention is to provide arelatively inexpensive, simple and effective method for case hardeningrefractory metals of the fourth group of the periodic table.

Another object of the invention is to provide a superior method for casehardening refractory metals consisting largely of titanium to form ahard adherent coating thereon.

A further object of the invention is to provide a superior method forforming a hard surface coating on refractory metals so as to enhance theresistance of the metal to wear, erosion, corrosion at high temperaturesand to overcome thetendency of titanium metal, in particular, to galland seize.

As indicated above, in its more common usage, the term case hardeningconnotes carburizing, that is to say the diflusion of gaseous carbonmonoxide at high temperatures into the surface of a ferrous metal beingtreated, followed by a heat treatment to form the hard case. As usedherein, the term case hardening shall be understood to mean the solidstate diffusion, into a refractory metal, of a solute speciescharacterized as a'metalloid atom or atoms of a metalloid compound of ametal from the fourth group of the periodic table, as for example themolecular carbides, borides, nitrides and silicides of titanium,zirconium and silicon respectively, which metalloid compounds ofthemselves produce a hard surface or case on the metal being casehardened, without the necessity for subsequent heat treatment.

While the mechanics of the action which takes place, by the methodof theinstant invention, may not be exactly understood, it is postulated that,in contradistinction to known processes of case hardening ferrous metalwherein gaseous carbon monoxide diffuses into the metal to form an ironcarbide which is subsequently converted to a harder composition such asMartensite by subsequent heat treatment, the metalloid compound used incarrying out the method of the instantinvention is itself an extremelyhard material, and during the heating period, the metalloid atoms ofthis hard material diffuse into the lattice structure of the metal beingtreated, thereby going into solid solution therewith to form a hardadherent coating on the surface of the metal without need of subsequentheat treatment.

Although this may not be the exact explanation of the reaction whichtakes place, in any event it has been discovered that by embedding arefractory metal, such as titanium or zirconium, in a tightly packedmetalloid compound powder of fine particle size, a hard case is formedon the refractory metal which penetrates deeply into the relatively softcore of the metal and adheres tenaciously thereto.

In its broadest aspects, therefore, this invention relate to a methodfor case hardening a refractory metal by embedding the refractory metalin a finely divided metalloid compound of a metal selected from thefourth group of the periodic .table, packing themetalloid compoundtightly around the refractory metal, and heating the embedded metal'at'a temperature and for a period of time sufficient to form a hardadherent coating thereon.

With reference to the type of metalloid compounds to be used asembedding materials, some degree of success has been achieved by the useof metalloid compounds currently available on the market. However, thesemetalloid compounds are usually prepared by grinding the product to therequired particle size, usually by mechanical means, and hence thecompounds are relatively impure and of non-uniform particle size. On theother hand, unusually consistent and superior results have been achievedby the use of metalloid compounds, the particles of which are of uniformsize and purity having, for example, a particle size Within the range offrom 0.5 to 10 microns; and which are prepared by calcining an admixtureof a hydrate of a refractory metal orsilica and carbon, with or withoutaddition agents, such'as' boric acid or nitrogen, depending upon thekind of metalloid desired, in accordance with the methods described inassignees copending applications Serial No. 307,721, filed September 3,1952; Serial No. 309,943, filed September 16, 1952;-and Serial No.397,470, filed December 10, 1953. Metalloid compounds of this type areof unusually uniform particle size, high purity and free of free carbonor other deleterious materials; and for purposes of identification arehereinafter referred to as hydrate-base metalloid compounds. 7

The case hardening process is carried out by embedding the refractorymetal to be case hardened in. one of the metalloid compound powdersselected from the group consisting of the carbides, nitrides or boridesof titanium, Zirconium or silicon, or mixtures thereof, packing themetalloid powder tightly around the metal, and

Patented Sept. 16, 1958 3 then heating the embedded metal for from about1 to 18 hours at a temperature within the range of from 700 to 1200 C.

At the end of the heating period the refractory metal is removed fromits metalloid compound bed and will be found to have acquired auniformly hard well-adhered coating, the thickness of which will bewithin the range of from 1 to mils, the hardness of the coating being ofthe order of 400-1200 Vickers.

While the method of practicing the invention, i. e. the techniqueemployed in embedding the refractory metal in the metalloid compoundpowder and heating the embedded metal is important to the achievement ofsuccessful results, the type of metalloid compound used is of greatersignificance for it has been found that a superior case is formed whenthe metal is embedded in a metalloid compound of high purity and ofrelatively uniform and fine particle size as is characteristic of thehydrate-base metalloid compounds referred to above. Moreover, since itis essential that the metalloid compound be packed tightly around themetal being treated, the fine particle size of the hydrate-basemetalloid compound is a distinct advantage. Conditions of temperatureand length of heating period may be varied to some degree depending uponthe nature of the case desired and the kind of refractory metal beingtreated as illustrated in the examples set forth below. In general, lowtemperatures produce a case substantially twice as hard as the core ofthe treated metal while high temperatures substantially quadruple thehardness of the core. Further, variations in the length of the heatingperiod at the same temperature produce cases of varying thicknesseshaving the same order of hardness. that it is not necessary to maintaina particular atmosphere over or around the metal being treated. Althoughan argon atmosphere was used in achieving the results set out in thetables below, it was found that similar results could be accomplished inany ordinary furnace atmosphere so long as the metalloid compound powderwas packed tightly around the specimen being treated.

In order to more fully illustrate the invention, a series of experimentswere made using metals of the fourth group of the periodic system andembedding these metals in the metalloid compounds identified above, ormixtures thereof. The results of these several experiments are tabulatedbelow.

TABLE I Case-hardening titanium 1 Micro- Thickness hardness MetalloidCompound Time and of Case, of Case,

Packing Temperature mils kg./mm. (Viekers) 18 hrs., 1,000 U 5 1,185 18hrs, 1,000 O--- 4 836 18 hrs., 1,000 O- 5 716 18 hrs., 1,000 C- 5 772 18hrs., 1,000 O. 5 770 1 Commercial grade.

TABLE II Case-hardening titanium 1 Micro- Metalloid Compound Time andThickness hardness Packing Temperature of Case, of Case,

mils kgJmrn. (V ickers) 6 hrs, 1,000 C 1-2 954 6 hrs., 1,000 O 1-2 946 6hrs, l.000 G A 1-2 694 6 hrs., 1,000 C 1-2 609 6 hrs., 1,000 O 1-2 752 1Commercial grade.

It is also significant TABLE III Case-hardening zirconium 1 Micro-Metalloid Compound Time and Thickness hardness Packing Temperature ofCase, of Case,

mils kg./mrn. (Vickers) 18 hrs., 1,000 C 1-2 627 18 hrs., 1,000 C 1-2554 ZrB; 18 hrs., 1,000 C 5 1, 254

1 Commercial grade.

TABLE IV Case-hardening titanium 1 Micro- Metalloid Compound Time andThickness hardness Packing Temperature of Case, of Case,

mils kg./mm. (Vickers) 18 hrs., 750 C 3 536 18 hrs, 750 C 3 416 1 hr.,1,200 0 1-2 870 1 hr., 1,200 C 1 810 1 Commercial grade.

As pointed out above, the invention is applicable to the refractorymetals of the fourth group, as for example titanium and zirconium andtheir alloys. nection it may be anticipated that the temperatures usedwhen treating alloys of titanium or zirconium may be somewhat diiferentthan those required for treating a substantially pure or commercialgrade metal. Moreover, it is not necessary to limit the packing materialto any one metalloid compound since mixed metalloids made up of any twoor more of the above mentioned metalloid compounds are also suitable.

By the improved process of this invention, it is possible to form hard,highly tenacious, relatively thick coatings on refractory metals by thecomparatively simple expedient of embedding the metal in a tightlypacked, relatively pure metalloid compound of uniform and relativelyfine particle size, and heating the embedded metal for a predeterminedlength of time and at a predetermined temperature, thereby to form acase on the refractory metal by solid state diffusion of the metalloidatoms therein.

The invention may be carried out in other specific ways than thoseherein set forth without departing from the spirit and essentialcharacteristics of the invention, and the present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all modifications coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

I claim:

1. A method for forming a hard adherent surface on a refractory metal,selected from the group consisting of titanium and zirconium comprising:embedding the refractory metal in a finely divided metalloid compoundselected from the group consisting of the carbides, nitrides and boridesof a metal of the fourth group of the periodic table, packing themetalloid compound tightly around the refractory metal, and heating therefractory metal therein for a period of time and at a temperaturesulficient to efiect solid state diffusion of a solute species of saidmetalloid compound into said refractory metal.

2. In a method for forming a hard, tough and adherent surface on a metalconsisting at least mainly of titanium, the improvement which comprises:embedding the metal in a hydrate-base metalloid compound selected fromthe group consisting of the carbides, nitrides and borides of a metalselected from the group consisting of titanium, zirconium, and siliconand mixtures thereof, the particle size of the metalloid compound beingin the range of In this confrom 0.5 to 10 microns, packing the saidmetalloid compound tightly around the metal, and heating the embeddedmetal therein for a period of time and at a temperature sufiicient toeffect solid state diffusion of a solute species of the said metalloidcompound into said refractory metal.

3. In a method for forming a hard, tough and adherent surface on a metalconsisting at least mainly of zirconium, the improvement whichcomprises: embedding the metal in a hydrate-base metalloid compoundselected from the group consisting of the carbides, nitrides and boridesof a refractory metal selected from the group consisting of titanium,zirconium, and silicon and mixtures thereof, the particle size of thesaid metalloid compound being in the range of from 0.5 to 10 microns,

packing the said metalloid compound tightly around the metal, andheating the embedded metal therein for a period of time and at atemperature suflicient to efiect solid state diffusion of a solutespecies of the said metalloid compound into said refractory metal.

4. In a process for case hardening refractory metals selected from thegroup consitsing of titanium and zirconium, the improvement comprising:embedding a refractory metal in a hydrate-base metalloid compoundselected from the group consisting of the carbide, boride and nitride ofa refractory metal selected from the group consisting of titanium,zirconium, silicon and mixtures thereof, the particle size of themetalloid being in the range of from 0.5 to 10 microns, packing the saidmetalloid compound tightly around said refractory metal, and heating therefractory metal therein for from 1 to 18 hours at a temperature in therange of from 700 to 1200 C. to effect solid state diffusion of a solutespecies of the said metalloid compound into said refractory metal.

References Cited in the file of this patent UNITED STATES PATENTS2,032,694 Gertler Mar. 3, 1936 2,190,050 Tracy Feb. 13, 1940 2,258,894Janco Oct. 14, 1941 2,592,414 Gibson Apr. 8, 1952 2,711,980 DeSantis etal. June 28, 1955

1. A METHOD FOR FORMING A HARD ADHERENET SURFACE ON A REFRACTORY METAL,SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND ZIRCONIUM COMPRISING:EMBEDDING THE REFRACTORY METAL IN A FINELY DIVIDED METALLOID COMPOUNDSELECTED FROM THE GROUP CONSISTING OF THE CARBIDES, NITRIDES AND BORIDESOF A METAL OF THE FOURTH GROUP OF THE PERIODIC TABLE, PACKING THEMETALLOID COMPOUND TIGHTLY AROUND THE REFRACTORY METAL, AND HEATING THEREFRACTORY METAL THEREIN FOR A PERIOD OF TIME AND AT A TEMPERATURESUFFICIENT TO EFFECT SOLID STATE DIFFUSION OF A SOLUTE SPECIES OF SAIDMETALLOID COMPOUND INTO SAID REFRACTORY METAL.